Summary Complete Notes for AQA A Level Biology

Biological Molecules
Monomers and Polymers

 Monomers are the smaller units from which larger molecules are made
 Polymers are made from a large number of monomers joined together
 Condensation reactions join two molecules together. A molecule of water is released
and a chemical bond is formed
 Hydrolysis reactions breaks a chemical bond between two molecules using a molecule
of water

Carbohydrates

 Monomers: monosaccharides
 Form glycosidic bonds
 Disaccharides are formed by the condensation of two monosaccharides:
o Glucose + glucose  maltose
o Glucose + fructose  sucrose
o Glucose + galactose  lactose
 Glucose has two isomers, alpha glucose and beta glucose:

,Polysaccharides

STARCH (alpha glucose)
 Function: store excess glucose for plants
 Structure: amylose and amylopectin:
o Amylose
o Long, unbranched chain  compact SO good for storage
o Angles of glycosidic bonds  coiled SO good for storage
o Amylopectin
o Long, branched chain  more access points for enzymes SO glucose can
be released quickly
o Insoluble in water so doesn’t affect water potential  SO no influx of
water by osmosis

GLYCOGEN (alpha glucose)
 Function: store excess glucose for animals
 Structure:
o Long, branched chain  SO more access points for enzymes so glucose can be
released quickly
o Compact SO good for storage

CELLULOSE (beta glucose)
 Function: forms cells walls in plants
 Structure:
o Long, unbranched chains of beta glucose
o Straight chains linked together by hydrogen bonds  form strong fibres called
microfibrils SO provide structural support for cell walls

Biochemical tests

BENEDICT’S TEST
JUST FOR NON-REDUCING SUGARS:
1. Add dilute HCl to sample
2. Heat sample
3. Add NaHCO3 to neutralise sample
FOR REDUCING AND NON-REDUCING SUGARS:
1. Heat sample with Benedict’s reagent (blue)
2. Forms coloured precipitate (intensity of colour change dependent on concentration of
reducing sugar, goes as far as brick-red)

IODINE TEST FOR STARCH
1. Add iodine dissolved in potassium iodide solution to sample
2. Sample changes from browny-orange to dark blue/black

,Lipids

TRIGLYCERIDES
 One glycerol (-OH) attached to three fatty acids (-OOH)
 Fatty acids have carboxylic acid functional group attached to hydrocarbon tail, which can
be saturated or unsaturated
 Ester bonds form by condensation reactions between glycerol and fatty acids
 Function: energy storage
o Hydrocarbon tails contain lots of chemical energy which is released when they
are broken down
o Insoluble in water so do not affect water potential and cause influx of water by
osmosis
o Bundle together as insoluble droplets because hydrocarbon tails are
hydrophobic so face inwards

PHOSHOLIPIDS
 One glycerol (-OH) attached to two fatty acids (-OOH) and one phosphate group
 Phosphate group is hydrophilic
 Fatty acid tails are hydrophobic
 Function: make up cell membrane phospholipid bilayer
o Heads face out towards water and tails face inwards  acts as barrier to water-
soluble substances

EMULSION TEST FOR LIPIDS
1. Shake test sample with ethanol for one minute
2. Pour solution into water
3. Formation of milky emulsion

, Proteins

 Monomers: amino acids
 Form peptide bonds by condensation reactions
 Two amino acids: dipeptide
 More than two amino acids: polypeptide
 One or more polypeptides: protein
 General structure (shared by all twenty amino acids):




 Primary structure: Sequence of amino acids in polypeptide chain
 Secondary structure: Hydrogen bonds form  coils/folds into alpha-helixes/beta-
pleated sheets
 Tertiary structure: Coils/folds further  formation of hydrogen bonds/ionic
bonds/disulfide bridges
 Quaternary structure: Several different polypeptide chains
 Proteins have a variety of functions: eg enzymes, antibodies, transport proteins,
structural proteins

Biuret test for proteins
 Add a few drops of NaOH
 Add some CuSO4 solution
 Solution will turn purple

Enzymes
 Enzymes are proteins that act as biological catalysts by providing an alternative route of
reaction with a lower activation energy
 They have an active site with a unique tertiary structure which allows it to bind with a
complementary substrate to form an enzyme-substrate complex
 Induced fit model: as the substrate binds, the active site changes shape slightly to fit
around it the right way




Nucleic Acids